Of the xylene isomers, i.e., ortho-, meta- and para-xylene, the paraxylene is of particular value as a chemical intermediate in a number of applications being useful in the manufacture of terephthalic acid which is an intermediate in the manufacturer of synthetic fibers. One process for manufacturing para-xylene is by disproportionation of toluene into xylenes. One of the disadvantages of this process is that large quantities of benzene are also produced. Another process used to obtain para-xylene involves the isomerization of a feedstream that contains non-equilibrium quantities of mixed ortho- and meta-xylene isomers and is lean with respect to para-xylene content. A disadvantage of this process is that the separation of the para-xylene from the other isomers is expensive.
Zeolites are comprised of a lattice of silica and optionally alumina combined with exchangeable cations such as alkali or alkaline earth metal ions. Although the term "zeolites" includes materials containing silica and optionally alumina, it is recognized that the silica and alumina portions may be replaced in whole or in part with other oxides. For example, germanium oxide, tin oxide, phosphorous oxide, and mixtures thereof can replace the silica portion. Boron oxide, iron oxide, gallium oxide, indium oxide, and mixtures thereof can replace the alumina portion. Accordingly, the terms "zeolite", "zeolites" and "zeolite material", as used herein, shall mean not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such silicon and aluminum, such as gallosilicates, silicoaluminophosphates (SAPO) and aluminophosphates (ALPO). The term "aluminosilicate zeolite", as used herein, shall mean zeolite materials consisting essentially of silicon and aluminum atoms in the crystalline lattice structure thereof.
Processes have been proposed for the production of xylenes by the methylation of toluene using a zeolite catalyst. For instance, U.S. Pat. No. 3,965,207 involves the methylation of toluene using a zeolite catalyst such as a ZSM-5. U.S. Pat. No. 4,670,616 involves the production of xylenes by the methylation of toluene using a borosilicate molecular sieve which is bound by a binder such as alumina, silica, or alumina-silica.
Synthetic zeolites are normally prepared by the crystallization of zeolites from a supersaturated synthesis mixture. The resulting crystalline product is then dried and calcined to produce a zeolite powder. Although the zeolite powder has good adsorptive properties, its practical applications are severely limited because it is difficult to operate fixed beds with zeolite powder. Therefore, prior to using in commercial processes, the zeohte crystals are usually bound.
The zeolite is typically bound by forming a zeolite aggregate such as a pill, sphere, or extrudate. The extrudate is usually formed by extruding the zeolite in the presence of a non-zeolitic binder and drying and calcining the resulting extrudate. The binder materials used are resistant to the temperatures and other conditions, e.g., mechanical attrition, which occur in various hydrocarbon conversion processes. Examples of binder materials include amorphous materials such as alumina, silica, titania, and various types of clays. It is generally necessary that the zeolite be resistant to mechanical attrition, that is, the formation of fines which are small particles, e.g., particles having a size of less than 20 microns.
Although such bound zeolite aggregates have much better mechanical strength than the zeolite powder, when such a bound zeolite is used for toluene methylation, the performance of the catalyst, e.g., activity, selectivity, activity maintenance, or combinations thereof, can be reduced because of the binder. For instance, since the amorphorous binder is typically present in an amount of up to about 50 wt. % of zeolite, the binder dilutes the adsorptive properties of the zeolite aggregate. In addition, since the bound zeolite is prepared by extruding or otherwise forming the zeolite with the binder and subsequently drying and calcining the extrudate, the amorphous binder can penetrate the pores of the zeolite or otherwise block access to the pores of the zeolite, or slow the rate of mass transfer to the pores of the zeolite which can reduce the effectiveness of the zeolite when used in hydrocarbon conversion processes. Furthermore, when such a bound zeolite is used in catalytic conversions processes such as toluene methylation, the binder may affect the chemical reactions that are taking place within the zeolite and also may itself catalyze undesirable reactions which can result in the formation of undesirable products.